The spatial and vertical distribution of phycoerythrin(PE)-containing chroococcoid cyanobacteria and the contribution of the <3 µm size fraction to overall phytoplankton chlorophyll a biomass were investigated in the central and southern North Sea during June and July 1989. PE-containing chroococcoid cyanobacteria cell numbers ranged between 6x105 and 4.4x107 cells·dm-3 and was typically between 0.5x107 and 3x107 cells·dm-3, lowest numbers being recorded near the British coast in central North Sea waters. The vertical distribution of these cyanobacteria showed no evidence to suggest a preferential accumulation deep in the euphotic zone. The <3 µm size fraction accounted for 6.6 to 57.5% of the total phytoplankton chlorophyll a biomass (mean 19.7%). In general, the relative significance of the <3 µm size fraction decreased with increasing total chlorophyll a biomass.